In optical communications systems information is transmitted by imparting temporal phase, frequency or intensity variations on an optical signal. The preferred form of modulation is governed by physical factors which include the type of the radiation source and the bandwidth of the communication system and economic factors such as the total system cost and the perceived number of units required.
From the perspective of overall cost the optical communications systems used by the telecommunications industry have the advantage that the capital cost burden of the system is shared by many subscribers. The expense of the materials, the fabrication process, and the packaging process are not necessarily the dominant concern for the system designer. The maturation and proven reliability of the systems and components developed for the telecommunications industry has however led to a number of new applications from a broader base of industries. In many of these newer applications it is desired to deliver products that utilize optical components to the home and office. In this environment the unit manufacturing cost of the discrete components becomes a critical factor in determining the viability of the product. Consequently severe constraints are imposed on the design and on the cost of the materials and technology employed.
The desk top printing and publishing business is one such application where the product design is driven by the need to minimize the total system cost. U.S. Pat. No. 5,371,817 issued Dec. 6, 1994 to Revelli et al describes a page scanning device whereby a single laser spot is divided into an array of individually addressable pixels through the use of branched single mode waveguides. As described by Revelli et al, the intensity of the radiation at each output pixel is controlled by an integrated optical waveguide modulator. The materials used to construct the page scanner are required to be inexpensive because the width of the recording media often exceeds eight inches. This precludes most of the commonly used electro-optic materials. A further requirement is that, in order to produce high quality hard copy of pictorial digital images, it is usually necessary for the printing device to be capable of defining at least 256 discrete tonal shades. Fewer output levels will normally cause artifacts which will reduce the overall image quality.
The limitations that are imposed on the design of integrated optical circuits by the need to use electro-optic materials is well known and has inspired research aimed towards the realization of an inexpensive optical modulator. With the advancement in the techniques used to process silicon there has been increasing interest in devices that utilize some form of micro-machining and micro-mechanical actuation to achieve the desired optical modulation: "Silicon as a Mechanical Material", K. E. Petersen, Proc. of the IEEE, Vol 70, No. 5, May 1982, pp. 420-457; and "Silicon Micro-machining for Micro-sensors and Microactuators", Benecke, W., Micro-electronic Eng., 11 (1990) 73-82. For example, U.S. Pat. No. 4,505,539 issued Mar. 19, 1985 to Auracher et al describes a device whereby the radiation propagating in a gap between two waveguides is modulated by a drop of liquid material. The liquid was moved into and out of gap by mechanical, electrical or magnetic forces. In another device, U.S. Pat. No. 5,024,500 issued Jun. 18, 1991 to Stanley et al, radiation from an input waveguide is switched between two output waveguides by the motion of a reflective micro-mechanical cantilever beam.
The application of micro-mechanics to the problem of optical modulation has been investigated extensively by W. Lukosz and P. Pliska: "Integrated Optical Interferometer as a Light Modulator and Microphone", Sensors and Actuators A, 25-27 (1991), pp. 333-340; U.S. Pat. No. 5,091,983 issued Feb. 25, 1992 to Lukosz; and W. Lukosz and W. Gabathuler, "Electro-Nanomechanically Tunable Integrated-Optical Bragg Reflectors", Integrated Photonics Research, 1993 Technical Digest Series Vol. 10, March 22-24, 1993, Palm Springs, Calif., USA, pp. 484-487. In U.S. Pat. No. 5,091,983, optical modulation is effected by inducing a near loss less change in the phase of the guided optical modes. This change in phase is then transformed into an intensity modulation by combining the phase shifted beam with a reference beam in a Mach-Zehnder interferometer or through the interference of two orthogonally polarized guided modes. The phase change is achieved by varying the thickness of an air gap located between the waveguide and a phase shifting element. The phase shifting element generally sits astride the waveguide and is moved through the application of mechanical, piezoelectric or electric forces.
A change in the phase and intensity of a mode propagating in an optical waveguide can also induced by changing the complex part of the propagation constant. In U.S. Pat. No. 5,278,925 to Boysel et al, a micromechanical membrance, made from a conductive medium, is used to effect a change in the absorptive properties of the optical waveguide. Boysel et al also disclosed a periodic array of conductive membrance modulators.
In U.S. Pat. No. 4,471,474 to Fields, a change of intensity is induced through a complex change in the propagation constant of a guided optical mode. However, the device described by Fields does not readily facilitate the fabrication of a plurality of modulators of varying length on a common substrate and is not therefore suitable for applications where a large number of channels and discrete output levels are required. In the device described by Fields the phase shifting media and the optical waveguide are formed on separate substrates and are then packaged in such a way that the phase shifting media is suspended above the optical waveguide.